Can I Do Mold Testing Myself?
The Simple Petri Dish Myth: Why DIY Mold Test Kits Can Mislead You (and Cost You More in the Long Run)
When people start worrying about mold in their homes, the first instinct is often to grab a cheap mold test kit from the hardware store. These kits-usually a simple Petri dish you leave out for a few days-promise quick answers for just a few dollars. Unfortunately, when it comes to mold testing, these DIY kits are more marketing gimmick than science.
Why DIY Mold Testing Fails
Petri dish or “settle plate” kits work by passively collecting spores that happen to fall onto the dish over time. But air doesn’t sit still-and neither does mold. Because these kits aren’t calibrated to measure an exact amount of air, the results are meaningless. A dish in a drafty room may collect far more spores than one in a closed area, even if the air quality is identical. Professional mold testing, by contrast, uses a calibrated air pump to pull a known volume of air through a specialized cartridge, giving measurable results in spores per cubic meter.
No Context = No Clarity
Even if a DIY kit shows mold growth, it tells you nothing about whether it’s a real indoor problem. Professionals compare indoor and outdoor samples to calculate what’s known as the Indoor/Outdoor (I/O) ratio. If indoor levels are significantly higher, that points to a hidden source of moisture or active mold growth indoors. Without this context, a Petri dish result is just a random number-no better than guessing.
The Risk of False Negatives
Another major flaw of DIY kits is that they only detect living mold spores. Many toxic species, such as Stachybotrys (black mold), don’t grow well on standard agar or may be nonviable when airborne. This means you could have a serious contamination problem and still get a “negative” test result.
What You Should Do Instead If you suspect a mold issue:
- Find and fix the moisture source-mold cannot grow without water.
- Inspect visually-if you see mold, there’s your answer.
- Call a professional for accurate mold testing using calibrated equipment and accredited labs.
